564 
ON ANIMAL HEAT. 
diminishes the vital action of the part it supplies, and conse- 
quently may only operate indirectly. Hunter remarks that the 
power of generating caloric cannot “depend upon the nervous 
system, for it is found in animals which have no brains or 
nerves.” (Animal CEcon., p. 104.) Acephalous foetuses have been 
often known to maintain their heat for several days. 
30. I will now proceed to detail a cause of animal caloric 
which appears to have been hitherto overlooked. It is founded 
on a discovery which I accidentally made, — that elastic substances, 
or bodies capable of reassuming their original shape when the 
power operating to extend them is removed, give out a quantity of 
caloric proportionate to the mechanical tension to which they are 
submitted. It is probable that this manifestation of heat by 
elastic bodies, when stretched, is only a modification of the ge- 
neral laws regulating the production of caloric during the changes 
of form in matter generally ; some sudden alteration of the posi- 
tion or shape of the component particles thus rendering heat sen- 
sible which was before latent. 
31. That elastic substances do thus eliminate heat when 
stretched, is proved by submitting a body so endowed, caout- 
chouc, for instance, to considerable tension, in close approxima- 
tion to the bulb of a sensible thermometer, when, in proportion 
to the power exercised in dilating the body, will be the quantity 
of caloric set free, and its consequent effect on the thermometer. 
It is also proved in a yet more familiar manner by grasping firmly 
the ends of a large strip of gum-elastic, and stretching it across 
the chin, when the heat evolved is immediately perceptible to the 
senses. 
32. This curious property, inherent in elastic substances, is 
supposed to have a material effect on the generation of animal 
warmth. It is well known that many of the component parts of 
the living frame are endowed with a considerable degree of elas- 
ticity, and it is but reasonable to suppose that, whenever this pro- 
perty is exercised within the body, the same physical phenomena 
result as are witnessed outside it. The tissue composing the arteries 
is of a highly elastic nature. The same property is also possessed 
by the trachea and bronchial tubes, though perhaps somewhat 
less in degree. When these properties are called into play, warmth 
is produced. On the contraction of the left side of the heart the 
column of blood occupying the arteries is suddenly increased, 
and a dilatation of their coats consequently ensues, forming the 
pulse. During every such dilatation of the arterial tissue through- 
out the body, a portion of caloric, governed by the power and 
extent of the expansion, is set free, and, communicating itself to 
the blood, spreads general warmth. The same thing occurs when 
